Pressurized gas is used in a number of applications ranging from carbonated beverage dispensers to oxygen systems used in medical and high-altitude environments. Pressurized gas is typically provided in pressurized tanks which release gas at a predetermined pressure through a regulator. When most of the gas has been dispensed from a particular tank, the gas pressure drops to a level below that which is necessary for the intended application. In the case of carbonated beverage dispensers, the beverages cease to be carbonated, i.e., becomes "flat". The tank is then typically replaced with a new, full tank of pressurized gas. However, manual tank replacement can be particularly inconvenient. For example, replacing an empty tank of carbon dioxide during lunch at a restaurant may be particularly inconvenient since no personnel may be available to effect the replacement. However, consumers generally prefer beverages carbonated and sales generally drop when carbonated beverages are not available in a particular dining establishment.
To address this problem, valves which automatically detect a reduction in pressure in a first tank and automatically switch to a second tank in response thereto have been developed. U.S. Pat. No. 5,014,733 to Wilson dated May 14, 1991 describes one such valve. The valve of the '733 patent uses mechanical devices to sense a drop in pressure and switches from a first supply tank to a second, full supply tank in response to the pressure drop.
To automatically trip the switching valve, the relatively high pressures of pre-regulated pressurized gas are required. Therefore, the switching value of the '733 patent is typically placed between the supply tanks and a regulator. Such generally has two primary drawbacks. The first is that the valve switches to the second supply tank when the first supply tank is not quite empty. Supply tanks generally store gas whose pressure can be as high as 850 psi and is regulated to about 100 psi for use in end-user equipment, e.g., carbonated beverage dispenser. Because of impression in mechanical switching mechanisms, conventional valves are adjusted to switch from the first supply tank to the second supply tank when the pressure of the gas in the first supply tank drops to about 200 psi. If such conventional valves were configured to switch at a lower pressure, the valve might not switch even if the pressure of the gas in the first supply tank dropped below 80 psi at which end-user equipment might become inoperative. However, even at 200 psi, gas in liquid form is still present in the first supply tank. Such premature discontinuation of use of the first supply tank costs the customer of the pressurized gas since a substantial amount of the gas is never used.
The second drawback of switching unregulated gas is that such conventional switching valves cannot be used with newer, bulk pressurized gas tanks which hold a much larger quantity of pressurized gas at a lower pressure, i.e., typically about 230 psi. Since such conventional valves are configured to switch from the first supply tank to the second supply tank when the pressure of the first tank drops below about 200 psi, such a conventional valve would switch before much of the pressurized gas in the bulk system is used. Such bulk systems require a much more precise switching mechanism.
Conventional switching valves such as that described in the '733 patent suffer from further disadvantages. For example, conventional valves only switch from the first supply tank to the second supply tank. When the pressure in the second supply tank drops below the threshold pressure, the valve must generally be manually reset to again supply pressurized gas from the first supply tank to the end-user equipment. By this time, the first supply tank which was previously empty hopefully has been replaced with a new first supply tank which is full. However, the user is not informed that the second supply tank is nearly empty and the end-user equipment must generally become inoperative before the user knows to replace the second supply tank. Unfortunately, it is also generally the first time the user is informed that the first supply tank is empty as well. Without careful observation by the user, the user is not informed that the valve has switched from the first supply tank to the second supply tank and, frequently, the first supply tank is not replaced until the second supply tank becomes empty. In this respect, such conventional automatically switching valves are no better than no switching valve at all except that the capacity of the first supply tank can be effectively doubled by adding the second supply tank.